1. Technical Field
The present disclosure relates to an actuator and an actuator set that stretch or contract in accordance with a change in temperature.
2. Description of the Related Art
In accordance with increasing demands for a machine (e.g., household robot) that works close to a human, there are increasing expectations for an artificial muscle actuator having features such as a light weight and flexibility like human muscles. There are various kinds of artificial muscle actuators. Most of the artificial muscle actuators utilize deformation of a high polymer material that is suitable for the features such as a light-weight and flexibility.
An actuator having a structure in which a twisted high polymer fiber bundle is wound in a coil is known as one of the actuators that utilize deformation of a high polymer material (see, for example, International Publication No. 2014/022667 and Carter S. Haines et al. “Artificial Muscles from Fishing Line and Sewing Thread”, SCIENCE, VOL. 343, pp. 868-872, 21 Feb. 2014). This actuator is a fibrous actuator that stretches and contracts in accordance with a change in temperature of the high polymer fiber bundle that is a plurality of twisted high polymer fibers. As the temperature of the high polymer fiber bundle rises, the twisted high polymer fibers are untwisted, and as a result, the entire length of the coil-like fibers contracts or stretches. As the temperature of the high polymer fiber bundle drops, the high polymer fibers are twisted again, and as a result, the entire length of the coil-like fibers stretches or contracts conversely. Whether the entire length of the coil-like fibers contracts or stretches when the temperature rises is determined on the basis of whether or not a direction of twisting of the high polymer fiber bundle is the same as a direction of winding of the coil. In a case where the direction of twisting of the high polymer fiber bundle is the same as the direction of winding of the coil, the entire length of the actuator contracts in accordance with a rise in temperature and stretches in accordance with a drop in temperature.
However, according to the conventional actuator, the temperature of the actuator need be kept constant in order to maintain a displaced state obtained by driving. This produces a problem that it is necessary to keep consuming energy in order to maintain the temperature although no work is done to the outside.